B3, default values, and implicit initialization

[Stephen Colebourne]

>From my perspective, the difference between B2 and B3 is vital, as I
fear developers will greatly overuse B3. I don't think "default
constructors" are the right focus.

The initial discussions of B2 vs B3 focussed on one main question -
does the type have a sensible default. `LocalDate` does not, but
`Decimal` or `Optional` does (zero or empty). The big issue Valhalla
faces during adoption from my perspective is the messaging, which is
far too easy to simplify to B3 faster than B2 faster than B1. The net
result would be many more 1979-01-01 type bugs. Without great care
here we could be creating the potential for many null-like
"million-dollar mistakes".

To counteract this, the syntax IMO needs to place the issue at hand
directly in the face of the developer. And the key question is "what
is the sensible default value for an instance of this type". Given
this, I think all authors of *all* value types should be forced to
*explicitly* define what the default value is. ie. it isn't something
where the language should choose one or the other as the default
(sic).

The obvious syntax is a field, which is implicitly public static
final. I don't feel that a class-level keyword is the right choice
here:

 public value LocalDate {
   default = null;
}
 public value Decimal {
   default = new;
}

In each case, the author has had to explicitly choose what the
sensible default is, and therefore implicitly chooses whether it is B2
and B3 - without any opportunity to be distracted by the performance
model. Neither B2 or B3 is chosen as the favourite by the language.
"It is a compilation error when a value class declaration does not
specify a default value".

By contrast, default constructors are one or two steps removed from
the actual decision point that the class author should actually be
thinking about, which is what the sensible default is. It is also the
case that the default constructor is never actually invoked, which
will be an ongoing point of surprise.

Terminology in specs just talks about what the default value is, eg
"authors should select the most appropriate default value for their
domain", "arrays are initialised to the default value of a value type"
or "if the default is null then ...":

The syntax is intended to make it perfectly reasonable to ask for
`LocalDate.default` or `Decimal.default` and get a sensible answer -
it looks like a "normal" constant in code. The use of `default = new`
by itself deliberately invokes the idea of a default constructor that
does nothing, without the need to spell it out.

Javadoc can be added to the `default` constant, which is very helpful.
For example it might include justification as to why LocaleDate does
not have a default value of 0000-01-01 or 1970-01-01.
Stephen


On Tue, 28 Mar 2023 at 20:13, Brian Goetz <brian.goetz at oracle.com> wrote:
>
> The recent update of JEP 401 contained a number of refinements to the user model, specifically, separating the primitive/reference distinction into a number of smaller distinctions (e.g., nullable vs non-nullable, optional vs required construction.)  Overall this has been a very positive step forward.
>
> We still have a need for the distinction between what we've been calling B2 and B3; JEP 401 currently frames that in terms of "construction is optional."  This is a big step forward; indeed, the key difference between them is whether the class _needs_ the "variables start out as null, and all instances are created by constructors" protection, or whether it admits the lighter-weight initialization protocol of "there's a a standard zero value, null-free variables are initialized to that" that primitives enjoy today.  (Note that B3 classes don't require this lighter protocol, they merely enable it, much as primitives all give you the option of boxing to get the full conservative initialization protocol.)
>
> The idea of framing this as "construction is optional" is a good one, but the expression of it proposed in JEP 401 feels "not quite there".  In this note I'll propose an alternative presentation, but the main goal here is around terminology and user model rather than syntax (so please keep the syntax agitation to a reasonable level.)
>
> The key distinction between B2 and B3 is that B3 has a _default value_ which the VM can summon at will.  This enables non-nullable heap variables to be flattened, because we can initialize these the same way we initialize other fields and array elements.  Further, that default value is highly constrained; it is a physical zero, the result of initializing all fields to their default value.
>
> Flattening is of course a goal, but it is not something that exists in the programming model -- its just an optimization.  What exists in the programming model is the default value, and what this unlocks is the possibility for variables to be _implicitly initializated_.  Reference-typed variables today are _explicitly initialized_; variables start out null and have to be initialized with a constructed value.  A class with a default value has the option (opted in through null-exclusion) for its variables to be implicitly initialized, which, like primitives, means that they start out with a valid default value, and can be further assigned to.
>
> Framed this way, the Valhalla performance story simplifies to:
>
>  - Give up identity, get flattening on the stack;
>  - Further give up explicit initialization, get flattening for small objects on the heap;
>  - Further give up atomicity, get flattening for larger objects on the heap.
>
> Giving up explicit initialization entails both the class opting out of explicit initialization, _and_ the variable opting out of nullity.
>
> The key new terminology that comes out of this is implicit vs explicit initialization.
>
>
> Syntactically, my preference is to indicate that the default value can be summoned by giving a value class a _default constructor_:
>
>     value class Complex {
>         public final double re, im;
>
>         public default Complex();
>     }
>
> A default constructor has no arguments, no body, no throws clause, and implicitly initializes all fields to their default values.  Unlike identity classes, value classes don't get constructions implicitly; a value class must declare at least one constructor, default or otherwise.  This replaces the idea of "optional constructor", which is a negative statement about construction ("but you don't have to call me"), with a more direct and positive statement that there is a _default constructor_ with the required properties.
>
> Note that this is similar to the existing concept of "default constructor", which you get for free in an identity class if you don't specify any constructors.  It is possible we can unify these features (and also with constructors in "agnostic" abstract classes), but first let's work out what it would mean in value classes, and see if we like it.
>
> In this model, a B3 class is just a value class with a default constructor -> a default constructor means that you have the choice of implicit or explicit initialization -> non-nullity at the use site opts into implicit initialization -> B3! gets flattening (for small layouts.)
>
>